docs: def stream_keys(self, bucket, timeout=None):
          """
          Streams keys from a bucket, returning an iterator that yields
          lists of keys.
          """
          msg_code = riak.pb.messages.MSG_CODE_LIST_KEYS_REQ
          codec = self._get_codec(msg_code)
          msg = codec.encode_stream_keys(bucket, timeout)
          self._send_msg(msg.msg_code, msg.data)
          return PbufKeyStream(self, codec)